An electrophotographic image forming apparatus, such as a laser printer and a digital copier, includes an optical scanning device configured to deflect a laser beam from a light source by using an optical deflector and expose an image carrier to the deflected laser beam. A color image forming apparatus includes a plurality of scanning optical systems corresponding to respective colors. To reduce the size of such an image forming apparatus, a technique in which a plurality of laser beams are made incident on a single optical deflector is employed (refer to PTL 1). In this case, in order to reduce the thickness of the optical deflector and to make it easier to separate the deflected light beams, the light beams are typically made incident on a deflection surface of the optical deflector at an angle to the sub-scanning direction. In addition, in consideration of the cost, the optical path lengths of the respective scanning optical systems are typically set to be equal to one another so that identical optical components can be used in the respective scanning optical systems. However, in order to set the optical path lengths of the respective scanning optical systems to be equal to one another, a scanning optical system disposed close to the optical deflector needs a plurality of reflection mirrors. When a reflection mirror is used, lines are likely to appear in an image due to dust or scratches on the mirror surface, and handing of scanning lines arising due to vibration of the mirror becomes an issue. Furthermore, as the number of mirrors increases, the configuration of the apparatus as a whole becomes more complex. Therefore, it is desirable that the optical scanning device be configured such that the number of reflection mirrors is kept to a minimum. Another technique is known, in which, even when the optical path lengths of respective scanning optical systems vary as the number of reflection mirrors is kept to a minimum, the degree of convergence of light beams incident on an optical deflector is varied, such that the size of the apparatus is reduced (refer to PTL 2).